Conventional on-board diagnostic systems monitor emission control devices, such as catalytic converters, oxygen sensors and nitrogen oxide (NOx) sensors, to ensure that regulatory emission standards are met. Based upon the on-board diagnostic system's information, an engine control unit determines the air/fuel ratio (“A/F ratio”) to maximize the engine's performance, e.g., maintaining an efficient control of fueling and combustion to ensure high conversion levels of nitrogen oxides, carbon monoxide, and hydrocarbons present in exhaust gas streams. In some regions of the world, gasoline mixtures contain sulfur, e.g., in an amount of approximately 300 to approximately 800 parts per million, as an additive, or, most likely, as an impurity. Catalytic converters that are continuously exposed to sulfur will perform below desired standards, e.g., efficiency and emission control, because the catalyst deposited on the catalyst substrate of the catalytic converter will become contaminated by the sulfur, i.e., experience “sulfur poisoning”.
Conventional on-board diagnostic systems for direct injection gasoline systems and lean burn systems monitor catalyst performance of emission control devices using a single sensor or a combination of sensors. The combination of sensors can monitor both the oxygen storage capacity and nitrogen oxide conversion efficiency of the catalysts. However, one drawback is that the sensors cannot monitor sulfur poisoning of the catalysts.
Another drawback is the insensitivity of the combination of sensors to accurately diagnose nitrogen oxide conversion efficiency. Typically, nitrogen oxide adsorber catalysts must include significant levels of oxygen storage capacity materials, and nitrogen oxide adsorber materials, to function effectively. However, nitrogen oxide adsorber materials possess a greater sensitivity to thermal deactivation than oxygen storage capacity materials. As a result, the thermal deactivation differential between the two materials reduces the accuracy of monitoring the oxygen storage capacity of the catalysts in the system. Since the nitrogen oxides storage capacity of a catalyst is used to diagnose its nitrogen oxides conversion efficiency, the thermal deactivation differential prevents an accurate diagnosis of the nitrogen oxide storage/conversion efficiency, and determination as to whether sulfur poisoning has occurred.
In contrast, on-board diagnostic systems for conventional diesel systems have not been used extensively; however, some diesel systems employ one or more thermistors or thermocouples to monitor temperatures within the exhaust system. The thermistors and/or thermocouples can be used to sense that temperatures within the diesel system are effective for operation and/or ensure the diesel system is protected from exposure to excessive operating temperatures. In addition, the thermistors and/or thermocouples can also be used to determine approximately when exothermic catalytic reactions, such as an oxidation reaction, taking place in a catalytic converter, decrease. This decrease can indicate that the catalyst's efficiency is decreasing, which signals the need to replace the catalytic converter. Consequently, emissions and catalyst performance for diesel systems are not monitored accurately and sulfur poisoning cannot be predicted.
Consequently, there exists a need for an apparatus and method for monitoring catalyst performance using on-board diagnostics in direct injection gasoline systems, lean burn systems and diesel systems.